Anaes · Applied cardiovascular & respiratory physiology
Context-sensitive half-time
Also known as CSHT · Context-sensitive half time · Recovery from infusion · Drug accumulation · Remifentanil CSHT · Ester metabolism
The context-sensitive half-time (CSHT) is the time for the plasma drug concentration to fall by 50 percent after stopping an infusion of a given duration, and it is the clinically relevant measure of recovery — far more useful than the terminal half-life for drugs given by infusion. The framework rests on five exam-critical ideas: the CSHT is NOT the same as the terminal elimination half-life, because during prolonged infusion the drug accumulates in the peripheral compartments (muscle, fat), and when the infusion stops the drug redistributes BACK from the periphery into the central compartment, slowing the decline in plasma concentration; for lipophilic drugs with a large Vd (fentanyl, thiopental), the CSHT increases dramatically with infusion duration — fentanyl CSHT rises from about 30 minutes after a 1-hour infusion to about 5 hours after 8 hours; for drugs with organ-independent ester metabolism (remifentanil), the CSHT is constant at about 3 to 5 minutes regardless of infusion duration — no accumulation occurs; the CSHT determines how quickly a patient will recover after stopping an infusion, and therefore which drugs are suitable for long cases (remifentanil, remimazolam) versus short cases only (fentanyl boluses are fine, but a fentanyl infusion is a poor choice for a case needing rapid emergence); and the CSHT is the pharmacokinetic rationale for the modern balanced anaesthetic technique — using a rapidly titratable, non-accumulating opioid (remifentanil) alongside a volatile or propofol for depth. Built on the remimazolam review (Dong 2025), the remifentanil postoperative pain study (Nagasaka 2014), the ultra-short-acting opioids review (Davis 2005), the paracetamol co-administration PK study (Raffa 2018), the opioids-in-organ-failure review (Hohne 2004), and the dexmedetomidine PK/TCI review (Tsai 2026).
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Red flags
- The CSHT is NOT the same as the terminal half-life. For fentanyl, the terminal half-life is about 3 to 4 hours, but the CSHT after an 8-hour infusion is about 5 hours — and after a short (1-minute) bolus it is just minutes. The CSHT captures the real-world recovery time.
- For lipophilic drugs with a large Vd (fentanyl, thiopental), the CSHT increases steeply with infusion duration — the peripheral compartments fill up during the infusion, then redistribute back into the central compartment when it stops, slowing the decline.
- Remifentanil has a CONSTANT CSHT of about 3 to 5 minutes regardless of infusion duration, because it is hydrolysed by non-specific plasma and tissue esterases (organ-independent clearance) — no accumulation in peripheral compartments. This is why it is the opioid of choice for long cases requiring rapid emergence.
- Remimazolam (a new benzodiazepine) also has organ-independent ester metabolism, giving a short and predictable CSHT — making it a potential alternative to propofol for procedural sedation and short anaesthetics.
- The CSHT explains why a fentanyl infusion is a poor choice for a long case needing rapid emergence — the CSHT rises to hours, and the patient will be opioid-sedated long after the surgery ends. Remifentanil is the answer.
Meet the patient
You finish an eight-hour pancreaticoduodenectomy. The patient ran on a fentanyl infusion throughout. Surgery ends. You turn off the infusion. Forty minutes later the patient is still not breathing. The trainee checks the drug card: terminal half-life of fentanyl, 3–7 hours. "Should be awake by now."[5]
It will not be. After eight hours, the peripheral compartments are loaded with fentanyl. When you stop, drug redistributes back into plasma and sustains the level — Höhne-exact: continuous infusion of fentanyl may result in accumulation and prolonged opioid effects (single-dose kinetics unaffected). The terminal half-life lied — the CSHT after a long fentanyl infusion is measured in hours, not minutes (hours-value standard teaching, not a number reported in the cited abstracts). That is the number that matters.[5]
References6ShowHide
- [1]Dong Y, Ma J. Remimazolam in anesthesia and sedation: A narrative review J Anesth Transl Med, 2025.PMID 41929379
- [2]Nagasaka Y, et al. [Efficacy and safety of remifentanil-based regimen for postoperative pain management in abdominal surgery patients: a double-blind study with low-dose remifentanil infusion of 0.02 microg x kg(-1) x min(-1)]. Masui, 2014.PMID 24783622
- [3]Davis PJ, Cladis FP. The use of ultra-short-acting opioids in paediatric anaesthesia: the role of remifentanil Clin Pharmacokinet, 2005.PMID 16029065
- [4]Raffa RB, et al. Pharmacokinetics of Oral and Intravenous Paracetamol (Acetaminophen) When Co-Administered with Intravenous Morphine in Healthy Adult Subjects Clin Drug Investig, 2018.PMID 29214506
- [5]Höhne C, Donaubauer B, Kaisers U [Opioids during anesthesia in liver and renal failure]. Anaesthesist, 2004.PMID 15074320
- [6]Tsai YF, et al. Dexmedetomidine Dosing Strategies in Sedation and Anesthesia: Pharmacokinetics, Safety, and Clinical Applications - A Narrative Review Drug Des Devel Ther, 2026.PMID 42232093